I. Field
The following description relates to pulse width modulated signal generation, in general, and phase-shifting pulse width modulated signal generation, in particular.
II. Background
The proliferation of light emitting diode (LED) backlighting applications has increased significantly with the onslaught of consumer portable devices such as cell phones and personal digital assistants. Phase-shifting pulse width modulation (PSPWM) power management integrated circuits can be employed to control an array of LEDs used for the backlighting applications. In conventional systems, a PSPWM circuit drives multiple strings of LEDs such that each LED string in an LED array turns on at different phases while maintaining the same duty cycle. As such, the electromagnetic interference is reduced relative to the electromagnetic interference that would result if the LED strings in the LED array are all turned on and off at the same time. Further, the power required to be provided to the LED array is reduced by turning the LED strings on at different phases, relative to the power required to be provided to the LED array if each of the LED strings was turned on at the same time. These conventional circuits typically and disadvantageously employ the two architectures discussed below in this section.
First, and as shown in FIG. 1, the circuit can include a PSPWM generator 110 configured to receive a pulse width-modulated signal 120 and output a plurality of LED driver signals 130a, 130b, . . . , 130x configured to drive LED strings (not shown) of an LED array. In some embodiments, LED driver signals 130a, 130b, . . . , 130x can be n LED driver signals. The PSPWM generator 110 can include a period measurement block 140 for measuring the period of the signal 120 and a plurality of programmable delay blocks 150b, . . . , 150x coupled to one another in series. In some embodiments, programmable delay blocks 150b, . . . , 150x can be n−1 programmable delay blocks. The PSPWM generator 110 can receive the signal 120 and the period measurement block 140 can determine the period of the signal, T. The period measurement block 140 outputs information indicative of the period to programmable delay blocks 150b, . . . , 150x. The programmable delay blocks 150b, . . . , 150x can output LED driver signals 130b, . . . , 130x. In some embodiments, the output LED driver signals 130b, . . . , 130x can be n−1 output LED driver signals. The PSPWM generator 110 also outputs output LED driver signal 130a, as shown in FIG. 1. For embodiments wherein there are n LED strings, and n LED drivers, the n LED driver signals 130a, 130b, . . . , 130x output from the PSWPM generator 110 can have a period T. The programmable delay blocks 150b, . . . , 150x can provide a staggered, delayed output of the LED driver signals 130b, . . . , 130x such that the pulses of the LED driver signals 130b, . . . , 130x are equally spaced relative to one another, and relative to LED driver signal 130a, as shown in FIG. 1. In some embodiments, the LED driver signals 130a, 130b, . . . , 130x are overlapping and, in other embodiments, non-overlapping. Unfortunately, this circuit disadvantageously requires a significant amount of memory and power because a programmable delay block (and corresponding power) is required for each of LED string in the LED array.
Second, the above-referenced circuit can utilize a counter-based architecture (not shown) that has two counters for each LED string for which power is to be provided. In the counter-based architecture, two counters can be substituted for each programmable delay block. The two counters identify the rising edge of the input signal, and a falling edge of the input signal, respectively. However, as with the circuit shown in FIG. 1, circuits employing counter-based approaches require a significant amount of power because at least two counters are required for each LED string in the LED array. Accordingly, circuits and methods resulting in lower power consumption and/or complexity than the convention approaches are desired. Further, due to the unique ability for hardware circuits to accurately implement the staggered, delayed aspect of the LED driver signals, all hardware circuits for automatic PSPWM signal generation are desired.